Nanoflares Keep Things Hot above the Sun

x-ray view of the Sun
X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO). This is the first picture of the sun taken by NuSTAR. The field of view covers the west limb of the sun. NuSTAR also detected x-rays from nanoflares, which are too small to be seen but are detectable by this instrument and others. NuSTAR/NASA-JPL/CalTech

One thing we all know about the Sun: it's incredibly hot. The surface (the outermost "layer" of the Sun that we can see) is 10,340 degrees Fahrenheit (F), and the core (which we can't see) is 27 MILLION degrees F. There's another part of the Sun that lies between the surface and us: it's the outermost "atmosphere", called the corona.It's some 300 times hotter than the surface. How can something farther away and out in space be hotter? You would think it would actually be cooling off the farther away it gets from the Sun. 

This question of how the corona gets so hot has kept solar scientists busy for a long time, trying to find an answer. It was once assumed that the corona heated gradually, but the cause of the heating was a mystery. 

The Sun is heated from within by a process called fusion. The core is a nuclear furnace, fusing atoms of hydrogen together to make atoms of helium. The process releases heat and light, which travel through the Sun's layers until they escape from the photosphere. The atmosphere, including the corona, lie above that. It should be cooler, but it's not. So, what could possibly heat the corona?

One answer is nanoflares. These are tiny cousins of the big solar flares that we detect erupting from the Sun. Flares are sudden flashes of brightness from the Sun's surface. They release incredible amounts of energy and radiation. Sometimes flares are also accompanied by massive releases of superheated plasma from the Sun called coronal mass ejections. These outbursts can cause what's called "space weather" (such as displays of northern and southern lights) at Earth and other planets.

Nanoflares are a different breed of solar flare. First, they erupt constantly, crackling along like countless little hydrogen bombs. Second, they are very, very hot, getting up to 18 million degrees Fahrenheit. That's hotter than the corona, which is usually a few million degrees F.  Think of them as a very hot soup, bubbling along on the surface of a stove, warming the atmosphere above it. With nanoflares, the combined heating of all those constantly blowing tiny explosions (which are as powerful as 10-megaton hydrogen bomb explosions) is likely why the coronosphere is so hot.  

The nanoflare idea is relatively new, and only recently have these little explosions been detected. The concept of nanoflares was first proposed in the early 2000s, and tested beginning in 2013 by astronomers using special instruments on sounding rockets. During the short flights, they studied the Sun, looking for evidence of these tiny flares (which are only a billionth of the power of a regular flare). More recently, the NuSTAR mission, which is a space-based telescope sensitive to x-rays, looked at the Sun's x-ray emissions and found evidence for the nanoflares. 

While the nanoflare idea seems to be the best one that explains coronal heating, astronomers need to study the Sun more in order to understand how the process works. They will watch the Sun during "solar minimum"—when the Sun is not bristling with sunspots that can confuse the picture. Then, NuSTAR and other instruments will be able to get more data to explain just how millions of tiny little flares going off just above the solar surface can heat the thin upper atmosphere of the Sun.